Field of the Invention
The present technology relates to a magnetic shape memory (MSM) alloys and to objects, such as elements, specimens or samples, for example actuators, wherein the objects are made of the alloy according to the present technology. The present invention also concerns methods of stabilizing mechanical and magneto-mechanical properties of objects (i.e. elements, specimens or samples) which comprise MSM alloys, as well as methods of producing magnetic shape memory alloy elements with stable mechanical and magneto-mechanical properties and especially with low twinning stress.
Description of Related Art
Ni—Mn—Ga magnetic shape memory (MSM) alloys are good candidates for actuating applications since they can exhibit magnetic field-induced strain of up to 10 percent with a rise time of less than 1 ms (MSM effect). The straining occurs due to rearrangement of martensite twin variants by motion of twin boundaries. Applications based on reverse mechanism have also been suggested including sensing, energy harvesting, and damping.
The actuating mechanism of MSM alloys has also a limitation related to mechanical hysteresis of twin boundary motion. The hysteresis can be described by twinning stress (TS), σTW, which has been reported to be in the range of 0.1-2 MPa. According to the model of MSM effect introduced by Likhachev and Ullakko [1] it follows that to obtain a reversible MSM effect, the twinning stress must be less than half of the magnetic stress, i.e. less than about 1.5 MPa because the maximum magnetic stress (σMAG) is about 3 MPa for 5-layered Ni—Mn—Ga martensite at room temperature.
In a typical magnetic actuator, the energy≈2·εσTW (where ε is the actuating strain) is dissipated during each actuating cycle. When the twinning stress is large, the overall performance of MSM material is low: considerable energy is dissipated leading to undesirable heating of the material, the applied magnetic field must be rather large to overcome high TS, output force and strain may be considerably smaller than the maximum theoretical values. These drawbacks make the exploitation of MSM alloys with high TS in actuators difficult or expensive.
Conversely, the closer the twinning stress is to zero, the higher is the efficiency of the MSM effect and the better actuating performance of the MSM material.